Various types of twist drills are known in the prior art. A listing of related prior art is delineated in the file history of the parent application to this case. However, the twist drills of known prior art suffer certain disadvantages. For example, twist drills tend to overheat during drilling operations which causes an expansion of the twist drill material resulting in additional binding forces being applied by the work piece against the cutting edges. This attenuates the speed which the twist drill can be fed into the work piece and also increases the wear on the drill itself.
Twist drills create shavings during the drilling operation which normally move from the tip to the shank of the drill through the flutes. The shaving sizes are related to the width of the leading cutting edge of the tip portion of the drill and can assume substantially large lengths with respect to the length of the drill. These shavings, particularly large shavings generated by certain twist drills, can become bound within the twist drill flutes between the margin or blade of the twist drill and the wall of the bore or hole in the work piece. Such bound shavings increase the work required to drill a hole and also reduce the useful life of the drill.
Therefore, it is an object of the present invention to provide an improved twist drill which has features which suit it for medical application and particularly for drilling bone as is required in certain orthopedic surgery procedures. Another object of the invention is to provide a twist drill which incorporates a modified tip having a groove which extends from the leading face to the trailing face of the tip portion of at least one of the cutting blades such that material being cut from the work piece initially passes through this groove prior to its being cut by the pass of the trailing cutting blade. In this connection, the shaving sizes are reduced and this feature can also be used to assist in equalizing the work done by the leading and trailing cutting blades.
This modified tip portion shaves the chips into narrow strips or chips which are received within a modified and enlarged operatively associated flute which serves to break the narrow strips into shorter lengths. The flute of the present invention is enlarged such that this greater flute cross-sectional area between the lands allows the chips moving through the flute to contact the wall of the work piece being drilled. This contact between the chips and the wall of the work piece serves to further break the chips up into smaller portions thereby enhancing the ability of the chips to move through the flutes. This feature reduces the friction between the twist drill and the work piece and thereby reduces the heat build up.
Still a further object of the present invention is to provide an improved twist drill in which the outer cutting edge of a portion of one of the blades forms an obtuse angle with the longitudinal axis of the drill which is greater than the angle formed by the further cutting edge of the further cutting blade.